1. Field of the Invention
The present invention relates to a cathode for a fuel cell, to a membrane electrode assembly, to a fuel cell and to a method of manufacturing proton-conductive inorganic oxides.
2. Description of the Related Art
With respect to the electrode for a fuel cell wherein a proton-conductive inorganic oxide and a proton-conductive organic polymer binder are employed, there has been known one comprising a proton-conductive inorganic oxide and a sulfuric acid-carrying metal oxide exhibiting solid exhibiting superacidity as described in JP-A 2004-158261(KOKAI) for example. The electrode of this kind is known as being unstable when it is used as a proton conductor for a fuel cell which produces water in the process of generating electricity, for a fuel cell which uses liquid fuel.
Meanwhile, the present inventors have proposed, as described in JP-A 2006-32287 (KOKAI), an electrode for a fuel cell, which comprises an oxide carrier using Ti, particles of oxide of W, and a binder for binding these materials. In an electrode catalyst layer formed of a composite constituted by an oxide (proton-conductive inorganic oxides) such as Ti, W, etc., a Pt catalyst and a binder, the continuity among the proton-conductive inorganic oxide as well as the continuity among the Pt catalyst carrier is obstructed due to the existence of the binder. Further, the binder that has adhered onto the surface of the proton-conductive inorganic oxide or the surface of the Pt catalyst obstructs the supply of water to the proton-conductive inorganic oxide. In such a case, due to the lack of water which is required for the production of protons, the supply of air to the Pt catalyst would become insufficient, thereby bringing about the shortage of three-phase interface needed for producing the electrode reaction.
In order to enhance the proton conductivity of the electrode catalyst layer and the electrode reaction, many efforts are now made on the development of the proton conductor. However, the performance of the proton conductor is still insufficient, leaving room for improvements.
Additionally, the conventional proton-conductive inorganic oxides have a great tendency that the nano-size particles of the proton-conductive inorganic oxides are caused to fuse to each other during the synthesis thereof, resulting in the growth of particles and hence in the decrease of specific surface area of the proton-conductive inorganic oxides, thereby reducing the useful proton-conducting site. Because of this, the proton conductivity decreases, so that it has been impossible to sufficiently increase the output of the fuel cell as a whole.